The FBI and U.S. Marshals Service are now involved in the search for a missing Waukesha County teen.

"She's been gone a few days. She's a minor, so we treat that very seriously, and that's why we're bringing in all resources to bear. Not just from FBI in Wisconsin, but national assets that are coming in today and tonight. We are assisting the inspector with the sheriff's office," FBI Special Agent in Charge Chris Combs said.

Kathryn Stalbaum, 15, texted a friend Tuesday morning that she would ride her bicycle to school after missing the bus. The teen has not been seen or heard from since.

"We don't have any immediate signs of foul play," he said. "We don't know what happened to Kathryn. We don't know if she left voluntarily. There's a lot we don't know, so we are not sure which details are important here. So I don't want to say the wrong thing, so we're really being very careful."

WISN 12 News has learned detectives traveled to Minneapolis to talk face-to-face with Stalbaum's adult sister there, who said she doesn't know where Kathryn is.

Investigators said they have been in regular contact with Stalbaum's biological parents.

Severson said Friday that Crime Stoppers of Waukesha County and the Waukesha Deputy Sheriff Association are offering a reward of up to $1,000 for information that leads to the safe return of Stalbaum.

Stalbaum is 5 feet 7 inches tall, has long brown hair and braces. She was last seen wearing a sweatshirt, jeans and pink Nike shoes.

Anyone with information about Stalbaum is asked to call the Waukesha County Sheriff's Department at 262-446-5090. Tips can also be submitted online at www.stopcrimewaukesha.com.

On Thursday, about three dozen firefighters joined Waukesha County sheriff's deputies in searching a roughly 4-square-mile area of the town of Genesee.

"It is out of character for her. She has a good relationship with her family. She's a communicative person who keeps her family apprised of what she's doing, so it is out of character for her to be out without having regular contact. She has not been a runaway before," Severson said.

The search included dogs and moving a dive team from a practice to a search of a nearby pond.

"We are pretty much on standby for whatever they need right now," Genesee Fire Department Chief Jim Moon said.

"So you're done searching?" WISN 12 News reporter Nick Bohr asked.

"We're done searching this section for now. They may come up with a different area to search at a later date. There's nothing right now that's going to change out here," Moon said.

But they are continuing to ask people to keep a lookout for Stalbaum.

"Just keep your eyes open -- and pray, I guess, would be the best thing," Moon said.

"This is a difficult case for us. We're really at a loss," Severson said.

Waukesha County authorities are checking social media and whether the Internet might provide any clues, but didn't want to comment on any exact details.

"We have collected videotapes from some businesses, so we are obviously looking over all those videos very carefully to see if we have any images of Kathryn," Severson said.

Investigators said if Stalbaum did leave on her own, they have a message for her.

"The message to Kathryn is that her family is very anxious to see her come home. She is in no trouble whatsoever. If she feels she is in trouble, it's nothing we can't work out," Severson said.

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